editor: Add horizontal scrollbar (#19495)

![editor_scrollbars](https://github.com/user-attachments/assets/76c26776-8fe4-47f8-9c79-9add7d7d2151)

Closes #4427 

Release Notes:

- Added a horizontal scrollbar to the editor panel
- Added `axis` option to `scrollbar` in the Zed configuration, which can
forcefully disable either the horizontal or vertical scrollbar
- Added `horizontal_scroll_margin` equivalent to
`vertical_scroll_margin` in the Zed configuration

Rough Edges:

This feature seems mostly stable from my testing. I've been using a
development build for about a week with no issues. Any feedback would be
appreciated. There are a few things to note as well:

1. Scrolling to the lower right occasionally causes scrollbar clipping
on my end, but it isn't consistent and it isn't major. Some more testing
would definitely be a good idea. [FIXED]
2. Documentation may need to be modified
3. I added an `AxisPair` type to the `editor` crate to manage values
that have a horizontal and vertical variant. I'm not sure if that's the
optimal way to do it, but I didn't see a good alternative. The `Point`
type would technically work, but it may cause confusion.

---------

Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>

docs/src/configuring-zed.md

@@ -534,7 +534,11 @@ List of `string` values
   "git_diff": true,
   "search_results": true,
   "selected_symbol": true,
-  "diagnostics": true
+  "diagnostics": true,
+  "axes": {
+    "horizontal": true,
+    "vertical": true,
+  },
 },
 ```
 
@@ -628,6 +632,41 @@ List of `string` values
 
 `boolean` values
 
+### Axes
+
+- Description: Forcefully enable or disable the scrollbar for each axis
+- Setting: `axes`
+- Default:
+
+```json
+"scrollbar": {
+  "axes": {
+    "horizontal": true,
+    "vertical": true,
+  },
+}
+```
+
+#### Horizontal
+
+- Description: When false, forcefully disables the horizontal scrollbar. Otherwise, obey other settings.
+- Setting: `horizontal`
+- Default: `true`
+
+**Options**
+
+`boolean` values
+
+#### Vertical
+
+- Description: When false, forcefully disables the vertical scrollbar. Otherwise, obey other settings.
+- Setting: `vertical`
+- Default: `true`
+
+**Options**
+
+`boolean` values
+
 ## Editor Tab Bar
 
 - Description: Settings related to the editor's tab bar.